We are trying to build a solar parabolic to offset our heating costs in the school. We are currently in the design and testing stage. We need some info on how we can build a system to track the sun. We are currently leaning towards a GPS setup or maybe photocell type where we could measure +,- voltage to keep the sun centered on our collector. Just some thoughts and lead ins would help.
How much for a basic light meter? Or hardwire in a piezoelectric cell (I think thatâs what Iâm thinking ofâŚ).
Wow, I had this project almost 30 years ago.
I used a cardboard tube with a light sensor on the bottom of it. The inside of the tube was covered with white paper to be semi-reflective. The microprocessor senses the intensity of the light coming down the tube from the sky and adjust the servos to move the solar parabolic for max exposure.
Use a simple looping routine to check sensor reading.
Good luck.
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For 1 degree of freedom tracking (1 axis that turns âleft or rightâ as the day progresses) you could have two light sensors with a slight angle offset (one pointed slightly east, the other slightly west). One will detect more light than the other. Simply mount the sensors on the solar array and repoint the solar array in the direction of more light until both cells detect equal amounts of light. This makes the assumption that the vector that symmetrically divides the angle between the two is perfectly orthogonal to the Earthâs surface at the sunâs peak.
â edit â itâs probably best to add some sort of light-blocking barrier (weather resilient) in between them too so that one will be put in the âshadowâ, guaranteeing that the other sees more light at some point, therefore moving the arrays. Should be a simple way to maximize output, if only by a few degrees worth of lumens over the whole array.
About 50 years ago an engineer at TI invented a 4 quadrant sensor, used for guided missles.
Same concept
http://www.kellrobotics.org/my_images/sensor.png
up/down difference = (sum A + B) - (sum C + D)
left/right difference = (sum A + C) - (sum B + D)
You need 4 light sensors
you could do the whole thing in analog electronics, op-amps and such,
OR
do it all in software using 4 A/D channels on a processor, like an old robot controller.
how sharp is the focus? If itâs not too sensitive, you could use a non-feedback system. Iâm assuming you have one axis of rotation. Just program what angle it needs to be at each time of day, have it move a bit when needed. The position will change over the year, so program that in as well.
What we are testing, for materials is a old TV dish 8â wide, we are using two motors one to control up down and one for left right.
Think about how much energy you need to harvest to make the effort worthwhile, and think about whether itâs more sensible to make several linear collectors, with only one direction of motion.
Silly thought. The position of the sun is pretty well-known. If youâre going to have a computer system to try to track the most intense light, why not just assume that pointing it directly at the sun is the best and program in the azimuth etc. and have the computer system aim it in the correct dimension based on time of day? Put an âNâ mark on your fixture to denote which side needs to be pointed to the north, and youâre pretty much good to goâŚ
To aim a dish âantennaâ at a celestial object, a Polar Mount is used. The track of the sun is quite predictable, an arc across the sky.
Imagine the front of a bicycle, the handlebar/front fork assembly. It turns in a single plane, left or right. If you mounted a dish to the front of the handlebars, youâd easily be able to scan across the horizon, right?
OK, now tilt the assembly backwards, so the dish points into the sky when the handlebars are straight ahead. But when you turn left or right, the dish points lower and lower until it looks at the horizon again - this happens both left and right.
This is called a polar mount. There is some error in pointing here, but for a solar collector it is certainly manageable. (If pointing error is unacceptable, youâd use an Azimuth-Elevation (Az-El) antenna rotator system, controlled bya computer. Readily available from Amateur Radio stores in the under-$1500 range)
You use a Linear Actuator to move the polar mount side to side. These are readily available from people who sell satellite TV antennas of 2 meters or larger. Kind of rare these days, what with DirecTV and such, but theyâre still out there. I saw a linear actuator in a surplus catalog for $40 recently. The actuator can be made to move slowly enough that it is always moving during the day, just really slowly, thus tracking the sun. At midnight, it runs for 2 or 3 minutes to reset the dish for the next sunrise.
The angle of the tube is simple: At the solstices the peak of the arc is at its highest or lowest, so normally one would pick an angle between the two, and careful observation and measurement shows that this equals your latitude. Funny how that worksâŚ
Here is a link to a Polar Mount installation instruction
Here is a Link to a Linear Actuator, just a good photo of one
OK, now that you have all thatâŚusing a dish like that is a bad idea.
The temperature at the focus of an 8 foot diameter dish will be enough to melt aluminum, possibly to melt steel if you are careful when applying your reflective coating. This is dangerous, and difficult to extract energy from. Maybe some engineering calculations are in order - Iâll leave that to you. But take a google at commercial solar collectors, see how THEY do it, and copy that.
In fact, you can make a linear system (a ling line-like collector) that doesnât need to move at all. Think of a rain gutter at the edge of a roof. Make a gutter thatâs a parabola, and put a tube right at the focus. Then, turn the assembly so it goes from the peak of the roof to the eaves. Then tilt it up at the magic angle (your latitude) and face it due South. This setup will be nearly as efficient as a tracking array, with half the cost (so you can aoofrd to put up more of them). Great for hot water generation. You need to use anti-freeze in those pipes if it gets cold where you are, though.
I think thatâs enough to get you started.
I agree that this could be dangerous. Take a look at this website to see that you could generate enough heat to melt aluminum.
ebarkerâs solution is the easiest and most reliable aiming device for this application. If the quadrant mechanism is tall enough or if the light sensors are close enough to the inside corners of the quad, then all four will be illuminated when the mechanism and/or the device to which it is mounted is pointing directly at the sun. It will continue to track the sun all day long, find the sun each morning or stop working when there is no sun or the there is clouds. A simple circuit could be used that would sense the intensity and then calibrated for disabling the system during times when there is not enough direct sunlight to efficiently heat the collector.
The aiming systems that Don is hinting at are known as lead screws and can be obtained for any size or power needs including moving multi ton radio telescopes up to several meters across. I have just finished refurbing a 4.5M dish that uses a lead screw driven by a right angle transmission fed by a 3/4 HP three phase motor. It can move the heated dish more than 45 degrees in a little over a minute. However, as Don discussed, these dishes when mounted in a polar mount can track a geostationary orbiting satellite. The sun however, is not geostationary and changes itâs declination (elevation) throughout the year, from solstice to solstice. For large collectors, two moving adjustments are needed, one for azimuth (east to west) and one for decilination (north to south). Of course, since one only needs to heat part of the year, the declination could be set to the average center of the sunâs apparent declination during the winter months and would not need nearly the azimuth arc during winter that it would need to track during the summer.
By the way, the surface of the dish you are using is textured to help reduce prime focus to prevent heating of the feed horn. The texture is selected to be large enough to efficiently reflect satellite energy (C & Ku Band) but not photons or infrared. A polished surface would be needed to produce effective heating. Aluminum foil, space blankets or something equally reflective would do the trick at least in the short term. As Don has pointed out, the heat can be extremely high so your system would need to remove the heat to prevent damage to the collector used at the dish focus. There are a number of books on the subject of solar collectors that would contain equations for predicting temperatures and suggestions for collection systems.